Schematic
This section contains the detailed electronic schematic of the Archean synthesizer. The schematic diagram is an essential resource for understanding how the circuit components are connected and how the overall system operates.
The schematic is available as a downloadable PDF file for convenient viewing and study. It visually represents the entire circuitry including:
- Power supply and voltage regulation
- Oscillators responsible for sound generation
- Filters shaping the sound spectrum
- Envelope generators controlling amplitude and modulation
- Control interfaces such as knobs, sensors, and buttons
- MIDI input/output and communication circuitry
Reading the schematic may seem complex at first, but it follows standard electronic symbols and conventions. By following the signal flow and component connections, you can gain insight into the design and functionality of the synthesizer.
We encourage users to reference the schematic if they want to repair, modify, or customize the hardware. Understanding the schematic alongside the source code will provide a comprehensive view of the Archean synthesizer’s architecture.
Please follow common safety precautions when working with electronic circuits. If you are new to electronics, consider studying basic schematic reading skills and circuit theory to make the most out of this resource.
This section contains the detailed electronic schematic of the Archean synthesizer. The schematic diagram is an essential resource for understanding how the circuit components are connected and how the overall system operates.
The schematic is available as a downloadable PDF file for convenient viewing and study. It visually represents the entire circuitry including:
- Power supply and voltage regulation
- Oscillators responsible for sound generation
- Filters shaping the sound spectrum
- Envelope generators controlling amplitude and modulation
- Control interfaces such as knobs, sensors, and buttons
- MIDI input/output and communication circuitry
Reading the schematic may seem complex at first, but it follows standard electronic symbols and conventions. By following the signal flow and component connections, you can gain insight into the design and functionality of the synthesizer.
We encourage users to reference the schematic if they want to repair, modify, or customize the hardware. Understanding the schematic alongside the source code will provide a comprehensive view of the Archean synthesizer’s architecture.
Please follow common safety precautions when working with electronic circuits. If you are new to electronics, consider studying basic schematic reading skills and circuit theory to make the most out of this resource.
Teensy 4.0
What is Teensy?
Teensy 4.0 is a small, powerful microcontroller development board created by PJRC (Paul Stoffregen).
Think of it as: A tiny computer optimized for real-time electronics projects.
Technical Specifications:
Processor
CPU: ARM Cortex-M7 (NXP iMXRT1062)
Speed: 600 MHz
Architecture: 32-bit ARM
One of the fastest microcontrollers available!
Memory
Flash (Program Storage): 2 MB
RAM: 1024 KB (1 MB)
Why Teensy 4.0 for Archean?
1. Processing Power
600 MHz = Fast enough for:
- 44 kHz audio generation
- Real-time DSP processing
- Multiple simultaneous tasks
- DMA operations
- Complex calculations
2. Audio Performance
Hardware features perfect for synthesis:
Precise Timers:
- IntervalTimer for accurate sample rate
- Microsecond precision
Fast ADC:
- 12-bit resolution
- Multiple channels
- DMA support
- High-speed sampling
Multiple SPI buses:
- Three DACs simultaneously
- Fast communication (20 MHz+)
3. Multiple Communication Buses
Archean uses:
SPI (3 buses available):
- DAC 1: Oscillator audio
- DAC 2: ADSR & Distance
- DAC 3: LFO & Element
I2C (3 buses available):
- Wire1: Touch sensors (MPR121 × 2)
- Wire1: Distance sensor (VL53L0X)
Serial MIDI:
- Hardware UART for traditional MIDI
USB:
- USB MIDI (computer connection)
- Programming interface
4. Arduino Compatibility
Teensy uses Arduino IDE:
- Familiar programming environment
- Huge library ecosystem
- Easy to learn
- Extensive community support
- Cross-platform (Windows, Mac, Linux)
5. Small Form Factor
Tiny size: Teensy 4.0: 36mm × 18mm
Perfect for:
- Portable instruments
- Eurorack modules
- Desktop synthesizers
- Embedded audio devices
Teensy 4.0 makes Archean possible:
- Fast enough - 44 kHz audio + real-time control
- Enough I/O - Multiple DACs, sensors, MIDI
- Real-time - Deterministic timing, no OS
- Professional - USB MIDI, low latency
- Compact - Tiny footprint
- Affordable - $20 for pro features
- Easy - Arduino IDE familiarity
- Powerful - DMA, multiple buses, fast ADC
Teensy 4.0 is a small, powerful microcontroller development board created by PJRC (Paul Stoffregen).
Think of it as: A tiny computer optimized for real-time electronics projects.
Technical Specifications:
Processor
CPU: ARM Cortex-M7 (NXP iMXRT1062)
Speed: 600 MHz
Architecture: 32-bit ARM
One of the fastest microcontrollers available!
Memory
Flash (Program Storage): 2 MB
RAM: 1024 KB (1 MB)
Why Teensy 4.0 for Archean?
1. Processing Power
600 MHz = Fast enough for:
- 44 kHz audio generation
- Real-time DSP processing
- Multiple simultaneous tasks
- DMA operations
- Complex calculations
2. Audio Performance
Hardware features perfect for synthesis:
Precise Timers:
- IntervalTimer for accurate sample rate
- Microsecond precision
Fast ADC:
- 12-bit resolution
- Multiple channels
- DMA support
- High-speed sampling
Multiple SPI buses:
- Three DACs simultaneously
- Fast communication (20 MHz+)
3. Multiple Communication Buses
Archean uses:
SPI (3 buses available):
- DAC 1: Oscillator audio
- DAC 2: ADSR & Distance
- DAC 3: LFO & Element
I2C (3 buses available):
- Wire1: Touch sensors (MPR121 × 2)
- Wire1: Distance sensor (VL53L0X)
Serial MIDI:
- Hardware UART for traditional MIDI
USB:
- USB MIDI (computer connection)
- Programming interface
4. Arduino Compatibility
Teensy uses Arduino IDE:
- Familiar programming environment
- Huge library ecosystem
- Easy to learn
- Extensive community support
- Cross-platform (Windows, Mac, Linux)
5. Small Form Factor
Tiny size: Teensy 4.0: 36mm × 18mm
Perfect for:
- Portable instruments
- Eurorack modules
- Desktop synthesizers
- Embedded audio devices
Teensy 4.0 makes Archean possible:
- Fast enough - 44 kHz audio + real-time control
- Enough I/O - Multiple DACs, sensors, MIDI
- Real-time - Deterministic timing, no OS
- Professional - USB MIDI, low latency
- Compact - Tiny footprint
- Affordable - $20 for pro features
- Easy - Arduino IDE familiarity
- Powerful - DMA, multiple buses, fast ADC
MIDI In
This circuit converts standard 5-pin MIDI signals to 3.3V logic levels for the Teensy 4.0.
Components:
R1 - Current Limiting Resistor (220Ω)
Purpose: Limits current through optocoupler LED
D2 - Protection Diode (1N4148)
Purpose: Protects against reverse polarity
Type: Fast switching diode
Function:
- Blocks negative voltage spikes
- Protects optocoupler LED
- No effect on normal operation (reverse biased)
Why needed: MIDI cables can pick up electrical noise
IC3 - Optocoupler (6N138)
What is an Optocoupler?
Two completely separate circuits connected only by light.
Why Use Optocoupler?
1. Electrical Isolation:
- Prevents ground loops
- Protects from voltage spikes
- Stops noise injection
2. Voltage Level Conversion:
- MIDI: 5V current loop
- Teensy: 3.3V logic
- Optocoupler bridges the gap
3. Safety:
- Protects expensive Teensy
- Isolates from faulty MIDI devices
- Standard MIDI specification requirement
R2 - Pull-up Resistor (220Ω)
Purpose: Pulls output HIGH when LED is off
Why needed: Phototransistor only pulls LOW, resistor provides HIGH
How It Works:
1. MIDI device sends data:
Transmitter sends: Note On (byte 0x90)
Current flows through MIDI cable
Arrives at Pin 4 and Pin 5
2. Current through LED:
Pin 5 (+5V) → R1 (220Ω) → LED (IC3 pin 2)
LED lights up when current flows
3. Light activates transistor:
LED emits light inside IC3
Phototransistor detects light
Transistor conducts (pulls output LOW)
4. Teensy receives signal:
Output pin 6 goes LOW
Teensy RX pin reads LOW
UART decodes MIDI byte
5. No signal:
No current → LED off
No light → Transistor off
Pull-up resistor → Output HIGH
Teensy reads HIGH (idle state)
Result: Safe, professional MIDI input that works with any MIDI device ever made!
Components:
R1 - Current Limiting Resistor (220Ω)
Purpose: Limits current through optocoupler LED
D2 - Protection Diode (1N4148)
Purpose: Protects against reverse polarity
Type: Fast switching diode
Function:
- Blocks negative voltage spikes
- Protects optocoupler LED
- No effect on normal operation (reverse biased)
Why needed: MIDI cables can pick up electrical noise
IC3 - Optocoupler (6N138)
What is an Optocoupler?
Two completely separate circuits connected only by light.
Why Use Optocoupler?
1. Electrical Isolation:
- Prevents ground loops
- Protects from voltage spikes
- Stops noise injection
2. Voltage Level Conversion:
- MIDI: 5V current loop
- Teensy: 3.3V logic
- Optocoupler bridges the gap
3. Safety:
- Protects expensive Teensy
- Isolates from faulty MIDI devices
- Standard MIDI specification requirement
R2 - Pull-up Resistor (220Ω)
Purpose: Pulls output HIGH when LED is off
Why needed: Phototransistor only pulls LOW, resistor provides HIGH
How It Works:
1. MIDI device sends data:
Transmitter sends: Note On (byte 0x90)
Current flows through MIDI cable
Arrives at Pin 4 and Pin 5
2. Current through LED:
Pin 5 (+5V) → R1 (220Ω) → LED (IC3 pin 2)
LED lights up when current flows
3. Light activates transistor:
LED emits light inside IC3
Phototransistor detects light
Transistor conducts (pulls output LOW)
4. Teensy receives signal:
Output pin 6 goes LOW
Teensy RX pin reads LOW
UART decodes MIDI byte
5. No signal:
No current → LED off
No light → Transistor off
Pull-up resistor → Output HIGH
Teensy reads HIGH (idle state)
Result: Safe, professional MIDI input that works with any MIDI device ever made!
Power Supply
Multi-rail power supply providing +15V, +12V, +5V for different circuit sections.
Input: 15V DC external supply
Outputs: +15V (raw), +12V (regulated), +5V (regulated)
Input Stage:
J1 - Power Jack
- Pin 1: +15V input
- Pin 3: Ground
D1 - Reverse Polarity Protection
Diode protects against backwards connection
FUSE - Overcurrent Protection
Protects entire circuit from shorts
Three Voltage Rails:
Rail 1: +15V Unregulated
Direct from input (after diode drop: ~14.3V)
No regulation: Raw supply voltage
Rail 2: +12V Regulated (IC1 - 7812)
Linear regulator: 15V → 12V
Components:
- C1, C2: Input filter
- C3: Output filter
Supplies:
- Op-amps (TL072, TL074)
- Analog circuits
Rail 3: +5V Regulated (IC2 - 7805T)
Linear regulator: 12V → 5V
Components:
- C4: Output decoupling
- C5 Additional filtering
Supplies:
- Teensy 4.0
- MPR121 touch sensors
- VL53L0X distance sensor
Rail 4: -12V Inverted Regulated
Linear regulator: 15V → -12V
Components:
- C5: Output filtering
Supplies:
- Op-amps (TL072, TL074)
- Analog circuits
Why This Design?
Cascaded Regulation
15V → 12V → 5V reduces heat per stage
Heat comparison:
Single stage 15V→5V:
(15-5)×0.5A = 5W (very hot!)
Two stages:
(15-12)×0.5A = 1.5W (IC1)
(12-5)×0.3A = 2.1W (IC2)
Total: 3.6W distributed
Multiple Rails
+15V: High-voltage analog (if needed for ±15V split supply)
+12V: Standard analog circuits
-12V: Standard analog circuits
+5V: Digital logic, Teensy
Capacitor Functions
Large electrolytics (47µF, 10µF):
- Filter low-frequency ripple
- Energy storage
- Stabilize regulators
Small ceramics (0.47µF, 0.1µF):
- Filter high-frequency noise
- Fast transient response
- Placed close to ICs
Multiple capacitors = clean power!
- Three voltage rails - 15V, 12V, 5V
- Cascade regulation - Distributed heat
- Protection - Reverse polarity + fuse
- Well-filtered - Multiple capacitor stages
- Isolated supplies - Separate regulators
Result: Stable, quiet power that keeps the Archean running reliably!
Input: 15V DC external supply
Outputs: +15V (raw), +12V (regulated), +5V (regulated)
Input Stage:
J1 - Power Jack
- Pin 1: +15V input
- Pin 3: Ground
D1 - Reverse Polarity Protection
Diode protects against backwards connection
FUSE - Overcurrent Protection
Protects entire circuit from shorts
Three Voltage Rails:
Rail 1: +15V Unregulated
Direct from input (after diode drop: ~14.3V)
No regulation: Raw supply voltage
Rail 2: +12V Regulated (IC1 - 7812)
Linear regulator: 15V → 12V
Components:
- C1, C2: Input filter
- C3: Output filter
Supplies:
- Op-amps (TL072, TL074)
- Analog circuits
Rail 3: +5V Regulated (IC2 - 7805T)
Linear regulator: 12V → 5V
Components:
- C4: Output decoupling
- C5 Additional filtering
Supplies:
- Teensy 4.0
- MPR121 touch sensors
- VL53L0X distance sensor
Rail 4: -12V Inverted Regulated
Linear regulator: 15V → -12V
Components:
- C5: Output filtering
Supplies:
- Op-amps (TL072, TL074)
- Analog circuits
Why This Design?
Cascaded Regulation
15V → 12V → 5V reduces heat per stage
Heat comparison:
Single stage 15V→5V:
(15-5)×0.5A = 5W (very hot!)
Two stages:
(15-12)×0.5A = 1.5W (IC1)
(12-5)×0.3A = 2.1W (IC2)
Total: 3.6W distributed
Multiple Rails
+15V: High-voltage analog (if needed for ±15V split supply)
+12V: Standard analog circuits
-12V: Standard analog circuits
+5V: Digital logic, Teensy
Capacitor Functions
Large electrolytics (47µF, 10µF):
- Filter low-frequency ripple
- Energy storage
- Stabilize regulators
Small ceramics (0.47µF, 0.1µF):
- Filter high-frequency noise
- Fast transient response
- Placed close to ICs
Multiple capacitors = clean power!
- Three voltage rails - 15V, 12V, 5V
- Cascade regulation - Distributed heat
- Protection - Reverse polarity + fuse
- Well-filtered - Multiple capacitor stages
- Isolated supplies - Separate regulators
Result: Stable, quiet power that keeps the Archean running reliably!
Voltage References
These circuits generate stable, precise reference voltages used throughout the Archean for calibration and accurate CV processing.
Three voltage references:
- VREF2.5 - 2.5V reference (from 3.3V supply)
- AREF-10V - 10V reference (from 12V supply)
- V_BIAS - 1V Bias voltage for analog circuits
Why Voltage References?
Problem with regular resistor dividers:
Supply voltage varies → Divided voltage varies
Temperature changes → Resistance changes
Result: Unstable, inaccurate reference
Solution with precision references:
Supply voltage varies → Reference stays constant
Temperature changes → Reference stays stable
Result: Accurate, stable reference
Critical for synthesizers:
- Pitch accuracy depends on stable references
- DAC output voltage determined by reference
- CV input scaling needs precision reference
Three voltage references:
- VREF2.5 - 2.5V reference (from 3.3V supply)
- AREF-10V - 10V reference (from 12V supply)
- V_BIAS - 1V Bias voltage for analog circuits
Why Voltage References?
Problem with regular resistor dividers:
Supply voltage varies → Divided voltage varies
Temperature changes → Resistance changes
Result: Unstable, inaccurate reference
Solution with precision references:
Supply voltage varies → Reference stays constant
Temperature changes → Reference stays stable
Result: Accurate, stable reference
Critical for synthesizers:
- Pitch accuracy depends on stable references
- DAC output voltage determined by reference
- CV input scaling needs precision reference
CV Inputs
This circuit is a signal conditioning stage for a Control Voltage (CV) input (typically from a modular synthesizer like Eurorack) to safely interface with an ADC pin on a Teensy 4.0 microcontroller.
Why is this circuit needed?
- Modular synthesizer CV signals are commonly bipolar (e.g., -5V to +5V, or sometimes wider ranges like -10V to +10V).
- The Teensy 4.0 ADC pins only accept 0V to 3.3V and are not tolerant of voltages outside this range (negative voltages or >3.3V would damage the pin).
- This circuit attenuates (scales down) the input signal, level-shifts it from bipolar to unipolar, buffers it for low-impedance drive, and provides basic protection.
Signal flow and stage analysis:
1. Input (J4 jack)
- CV signal enters through the tip.
- R6 provides current limiting and basic overvoltage protection.
2. First op-amp stage (IC5D, MCP6004)
- Configured as a non-inverting amplifier with a deliberate offset.
- Non-inverting input (+) receives a DC bias voltage derived from the +10V reference (AREF-10V) through R10 to a virtual ground node, combined with the input signal path.
- Feedback network: R8 from output to inverting input, R7 from inverting input to ground.
- Gain of this stage = 1 + R8/R7 = 1 + 12k/24k = 1.5.
- The DC bias at the non-inverting input provides the level shift.
3. Bias/offset generation
- AREF-10V (+10V reference) → R10 → node that feeds the non-inverting input.
- This creates a stable positive offset that shifts the entire signal upward.
4. Second op-amp stage (labeled ICA4, also MCP6004)
- Configured as a non-inverting attenuator/buffer.
- Gain = 1 + R9 / (effective resistance to ground), but looking at the schematic, the input comes through a divider involving previous stage and R9 in feedback.
- More precisely, this stage further scales the signal down.
- R11 in series with the output acts as a small damping resistor to improve stability and limit current into the ADC pin.
5. Capacitors
- C6 (likely 47pF or similar) and other small caps provide high-frequency filtering and prevent oscillation.
Overall function and scaling:
With the updated resistor values, the circuit performs the following transformation:
- Attenuates the incoming CV amplitude (total gain < 1 overall).
- Adds a DC offset of approximately 1.65V (half of 3.3V).
- Maps a typical -5V to +5V CV input range to approximately 0V to 3.3V at the output.
- For wider input ranges (e.g., -10V to +10V), the signal will be compressed or clipped slightly, but the high input resistance and attenuation prevent damage.
Key advantages:
- Protection: Series resistors (R6, R11) limit current.
- Accuracy: Dual op-amp buffering ensures very low output impedance for the Teensy ADC (improves conversion accuracy and speed).
- Stability: MCP6004 is a rail-to-rail, low-power quad op-amp ideal for single-supply (3.3V) operation.
- Noise filtering: Small capacitors reduce high-frequency noise.
Result: this is a robust, standard design for safely reading bipolar analog control voltages from modular synthesizers into a Teensy 4.0 (or similar 3.3V microcontroller) ADC. It attenuates, level-shifts, buffers, and protects the microcontroller input while preserving good fidelity for pitch or parameter control applications (e.g., DIY oscillators, sequencers, or quantizers). If your CV source uses a different voltage range, minor resistor tweaks can optimize the full-scale mapping.
Why is this circuit needed?
- Modular synthesizer CV signals are commonly bipolar (e.g., -5V to +5V, or sometimes wider ranges like -10V to +10V).
- The Teensy 4.0 ADC pins only accept 0V to 3.3V and are not tolerant of voltages outside this range (negative voltages or >3.3V would damage the pin).
- This circuit attenuates (scales down) the input signal, level-shifts it from bipolar to unipolar, buffers it for low-impedance drive, and provides basic protection.
Signal flow and stage analysis:
1. Input (J4 jack)
- CV signal enters through the tip.
- R6 provides current limiting and basic overvoltage protection.
2. First op-amp stage (IC5D, MCP6004)
- Configured as a non-inverting amplifier with a deliberate offset.
- Non-inverting input (+) receives a DC bias voltage derived from the +10V reference (AREF-10V) through R10 to a virtual ground node, combined with the input signal path.
- Feedback network: R8 from output to inverting input, R7 from inverting input to ground.
- Gain of this stage = 1 + R8/R7 = 1 + 12k/24k = 1.5.
- The DC bias at the non-inverting input provides the level shift.
3. Bias/offset generation
- AREF-10V (+10V reference) → R10 → node that feeds the non-inverting input.
- This creates a stable positive offset that shifts the entire signal upward.
4. Second op-amp stage (labeled ICA4, also MCP6004)
- Configured as a non-inverting attenuator/buffer.
- Gain = 1 + R9 / (effective resistance to ground), but looking at the schematic, the input comes through a divider involving previous stage and R9 in feedback.
- More precisely, this stage further scales the signal down.
- R11 in series with the output acts as a small damping resistor to improve stability and limit current into the ADC pin.
5. Capacitors
- C6 (likely 47pF or similar) and other small caps provide high-frequency filtering and prevent oscillation.
Overall function and scaling:
With the updated resistor values, the circuit performs the following transformation:
- Attenuates the incoming CV amplitude (total gain < 1 overall).
- Adds a DC offset of approximately 1.65V (half of 3.3V).
- Maps a typical -5V to +5V CV input range to approximately 0V to 3.3V at the output.
- For wider input ranges (e.g., -10V to +10V), the signal will be compressed or clipped slightly, but the high input resistance and attenuation prevent damage.
Key advantages:
- Protection: Series resistors (R6, R11) limit current.
- Accuracy: Dual op-amp buffering ensures very low output impedance for the Teensy ADC (improves conversion accuracy and speed).
- Stability: MCP6004 is a rail-to-rail, low-power quad op-amp ideal for single-supply (3.3V) operation.
- Noise filtering: Small capacitors reduce high-frequency noise.
Result: this is a robust, standard design for safely reading bipolar analog control voltages from modular synthesizers into a Teensy 4.0 (or similar 3.3V microcontroller) ADC. It attenuates, level-shifts, buffers, and protects the microcontroller input while preserving good fidelity for pitch or parameter control applications (e.g., DIY oscillators, sequencers, or quantizers). If your CV source uses a different voltage range, minor resistor tweaks can optimize the full-scale mapping.
DAC for Oscillator
This circuit generates the main audio output from digital samples, converting Teensy's digital waveform data into analog audio voltage.
Purpose: High-speed DAC (Digital to Analog Converter) with filtering for clean audio output.
Stage 1: SPI Communication from Teensy
Teensy Connections
SPI signals:
- CS-TENNSY-OSC: Chip Select for oscillator DAC
- SCK-TENNSY-OSC: SPI Clock
- MOSI-TENNSY-OSC: Master Out Slave In (data)
How it works:
1. Teensy pulls CS low (select this DAC)
2. Teensy sends 16-bit word via MOSI
3. Clock pulses on SCK
4. DAC latches data and updates output
5. Teensy pulls CS high (deselect)
Stage 2: DAC Chip (IC11 - MCP4921)
MCP4921 Specifications
Type: 12-bit voltage output DAC
Key features:
- 12-bit resolution (4096 steps)
- SPI interface (20 MHz capable)
- Internal 2.048V reference
- Single channel
- Rail-to-rail output
- Low glitch energy
Stage 3: Voltage Reference (VREF2.5)
Reference Voltage Generation
VREF2.5 = 2.5V precision reference
Why needed?
- DAC output range determined by VREF
- Stability = pitch stability
- Low noise = clean audio
Stage 4: Output Buffer (IC12A - TL072)
TL072 Op-Amp Configuration
Type: Dual JFET input op-amp
Configuration: Voltage follower (unity gain buffer)
C26 - High-Frequency Filter (470pF)
Forms RC low-pass filter with R51:
Cutoff frequency:
fc = 1 / (2π × R × C)
fc = 1 / (2π × 18kΩ × 470pF)
fc = 1 / (53.1 × 10⁻⁶)
fc ≈ 18.8 kHz
Purpose:
- Removes DAC switching artifacts
- Filters SPI clock feedthrough
- Reduces high-frequency noise
- Still passes full audio range (20 Hz - 20 kHz)
C27 - AC Coupling Capacitor (10µF)
Purpose: Blocks DC, passes AC audio signal
Why needed?
- DAC output has DC component (0-2V range)
- Audio circuits expect AC signals (centered at 0V)
- Removes DC offset
Coupling action:
DAC output: 0-2V DC (varies with audio)
After C27: AC signal centered at 0V
Size: 10µF allows low frequencies to pass (down to ~16 Hz)
Why Buffer the DAC?
DAC → Directly to next stage
DAC must provide all current
Output voltage sags under load
Distortion and noise
Solution with buffer:
DAC → Buffer → Next stage
DAC provides minimal current
Buffer provides drive current
Clean, strong output signal
Stage 5: Second Buffer with Gain (IC12B)
TL072 Op-Amp (Channel B)
Configuration: Non-inverting amplifier
C28 - Input Coupling (10µF)
Purpose: AC coupling to second stage
Ensures: No DC offset into amplifier
R52 - Input Resistor (24.9kΩ)
Purpose:
- Sets input impedance
- Part of gain network
- Provides DC path for op-amp
R53 - Feedback Resistor (100kΩ)
Purpose: Sets amplifier gain
Gain calculation:
Gain = 1 + (R53 / R52)
Gain = 1 + (100k / 24.9k)
Gain = 1 + 4.02
Gain ≈ 5×
Why amplify?
- DAC output: 0-2V range
- After amplification: 0-10V range
- Better signal-to-noise ratio
- Matches modular synth levels
C29 - Frequency Compensation (100pF)
Placement: Parallel with R53 (feedback resistor)
Purpose:
- Prevents high-frequency oscillation
- Limits bandwidth (stability)
- Reduces noise gain at high frequencies
Cutoff frequency:
fc = 1 / (2π × R53 × C29)
fc = 1 / (2π × 100kΩ × 100pF)
fc ≈ 15.9 kHz
Result:
- Audio frequencies amplified (20 Hz - 15 kHz)
- Ultrasonic frequencies attenuated
- Stable, clean amplification
Purpose: High-speed DAC (Digital to Analog Converter) with filtering for clean audio output.
Stage 1: SPI Communication from Teensy
Teensy Connections
SPI signals:
- CS-TENNSY-OSC: Chip Select for oscillator DAC
- SCK-TENNSY-OSC: SPI Clock
- MOSI-TENNSY-OSC: Master Out Slave In (data)
How it works:
1. Teensy pulls CS low (select this DAC)
2. Teensy sends 16-bit word via MOSI
3. Clock pulses on SCK
4. DAC latches data and updates output
5. Teensy pulls CS high (deselect)
Stage 2: DAC Chip (IC11 - MCP4921)
MCP4921 Specifications
Type: 12-bit voltage output DAC
Key features:
- 12-bit resolution (4096 steps)
- SPI interface (20 MHz capable)
- Internal 2.048V reference
- Single channel
- Rail-to-rail output
- Low glitch energy
Stage 3: Voltage Reference (VREF2.5)
Reference Voltage Generation
VREF2.5 = 2.5V precision reference
Why needed?
- DAC output range determined by VREF
- Stability = pitch stability
- Low noise = clean audio
Stage 4: Output Buffer (IC12A - TL072)
TL072 Op-Amp Configuration
Type: Dual JFET input op-amp
Configuration: Voltage follower (unity gain buffer)
C26 - High-Frequency Filter (470pF)
Forms RC low-pass filter with R51:
Cutoff frequency:
fc = 1 / (2π × R × C)
fc = 1 / (2π × 18kΩ × 470pF)
fc = 1 / (53.1 × 10⁻⁶)
fc ≈ 18.8 kHz
Purpose:
- Removes DAC switching artifacts
- Filters SPI clock feedthrough
- Reduces high-frequency noise
- Still passes full audio range (20 Hz - 20 kHz)
C27 - AC Coupling Capacitor (10µF)
Purpose: Blocks DC, passes AC audio signal
Why needed?
- DAC output has DC component (0-2V range)
- Audio circuits expect AC signals (centered at 0V)
- Removes DC offset
Coupling action:
DAC output: 0-2V DC (varies with audio)
After C27: AC signal centered at 0V
Size: 10µF allows low frequencies to pass (down to ~16 Hz)
Why Buffer the DAC?
DAC → Directly to next stage
DAC must provide all current
Output voltage sags under load
Distortion and noise
Solution with buffer:
DAC → Buffer → Next stage
DAC provides minimal current
Buffer provides drive current
Clean, strong output signal
Stage 5: Second Buffer with Gain (IC12B)
TL072 Op-Amp (Channel B)
Configuration: Non-inverting amplifier
C28 - Input Coupling (10µF)
Purpose: AC coupling to second stage
Ensures: No DC offset into amplifier
R52 - Input Resistor (24.9kΩ)
Purpose:
- Sets input impedance
- Part of gain network
- Provides DC path for op-amp
R53 - Feedback Resistor (100kΩ)
Purpose: Sets amplifier gain
Gain calculation:
Gain = 1 + (R53 / R52)
Gain = 1 + (100k / 24.9k)
Gain = 1 + 4.02
Gain ≈ 5×
Why amplify?
- DAC output: 0-2V range
- After amplification: 0-10V range
- Better signal-to-noise ratio
- Matches modular synth levels
C29 - Frequency Compensation (100pF)
Placement: Parallel with R53 (feedback resistor)
Purpose:
- Prevents high-frequency oscillation
- Limits bandwidth (stability)
- Reduces noise gain at high frequencies
Cutoff frequency:
fc = 1 / (2π × R53 × C29)
fc = 1 / (2π × 100kΩ × 100pF)
fc ≈ 15.9 kHz
Result:
- Audio frequencies amplified (20 Hz - 15 kHz)
- Ultrasonic frequencies attenuated
- Stable, clean amplification
CV Output DACs
Dual-channel DAC generating two independent control voltage outputs: Element CV and LFO CV.
Purpose: Convert digital modulation data to analog CV signals for external patching.
Stage 1: MCP9422 Dual DAC (IC8)
DAC Chip
Type: 12-bit dual-channel voltage output DAC
Key features:
- Two independent channels (DACA and DACB)
- 12-bit resolution (4096 steps per channel)
- SPI interface (20 MHz capable)
- Internal voltage reference
- Simultaneous or independent updates
- Supply: +3.3V
Channel A: Element CV Output (IC9B)
Buffer Stage (IC9B - TL072P)
Configuration: Unity gain voltage follower
Op-amp pins:
- Pin 5 (+): Input from DACB
- Pin 6 (-): Feedback from output
- Pin 7 (Output): To ELEMENT_CV_OUT jack
Signal Path Components
R34 (24.9kΩ):
- Connects input to V_BIAS (1.0V)
- Provides DC reference point
- High impedance (doesn't load V_BIAS)
C21 (470pF):
- High-frequency filter capacitor
- Removes switching artifacts
- Connected in feedback loop
R35 (100kΩ):
- Feedback resistor
- Sets unity gain configuration
- Determines frequency response with C21
R36 (220Ω):
- Output current limiting resistor
- Short circuit protection
- Impedance matching
Jack (J9): ELEMENT_CV_OUT
- 3.5mm output jack
- Connects to external modular gear
Cutoff Frequency
R35 + C21 form low-pass filter:
fc = 1 / (2π × 100kΩ × 470pF)
fc ≈ 3.4 kHz
Purpose: Smooth CV changes, remove high-frequency noise
Channel B: LFO CV Output (IC9A)
Buffer Stage (IC9A - TL072P)
Configuration: Unity gain voltage follower
Op-amp pins:
- Pin 3 (+): Input from DACA
- Pin 2 (-): Feedback from output
- Pin 1 (Output): To LFO_CV_OUT jack
Signal Path Components
Identical architecture to Channel A:
R31 (24.9kΩ): V_BIAS connection
C20 (0.1µF): AC coupling capacitor
- Larger than Channel A (100nF vs 470pF)
- Passes lower frequencies
- Better for slow LFO sweeps
C19 (470pF): High-frequency filter
R32 (100kΩ): Feedback resistor
R33 (220Ω): Output protection
How It Works:
Independent Channel Operation
Element CV (Channel A):
1. Teensy sends data via SPI
2. MCP9422 updates DACB output
3. Voltage: 0 to 2.5V
4. Buffer (IC9B) isolates and filters
5. Output: Clean CV at ELEMENT_CV_OUT jack
LFO CV (Channel B):
1. Teensy sends data via SPI (different command)
2. MCP9422 updates DACA output
3. Voltage: 0 to 2.5V
4. Buffer (IC9A) isolates and filters
5. Output: Clean CV at LFO_CV_OUT jack): LFO_CV_OUT
Result: Professional CV outputs for expanding the Archean with external modular gear - endless patching possibilities!
Purpose: Convert digital modulation data to analog CV signals for external patching.
Stage 1: MCP9422 Dual DAC (IC8)
DAC Chip
Type: 12-bit dual-channel voltage output DAC
Key features:
- Two independent channels (DACA and DACB)
- 12-bit resolution (4096 steps per channel)
- SPI interface (20 MHz capable)
- Internal voltage reference
- Simultaneous or independent updates
- Supply: +3.3V
Channel A: Element CV Output (IC9B)
Buffer Stage (IC9B - TL072P)
Configuration: Unity gain voltage follower
Op-amp pins:
- Pin 5 (+): Input from DACB
- Pin 6 (-): Feedback from output
- Pin 7 (Output): To ELEMENT_CV_OUT jack
Signal Path Components
R34 (24.9kΩ):
- Connects input to V_BIAS (1.0V)
- Provides DC reference point
- High impedance (doesn't load V_BIAS)
C21 (470pF):
- High-frequency filter capacitor
- Removes switching artifacts
- Connected in feedback loop
R35 (100kΩ):
- Feedback resistor
- Sets unity gain configuration
- Determines frequency response with C21
R36 (220Ω):
- Output current limiting resistor
- Short circuit protection
- Impedance matching
Jack (J9): ELEMENT_CV_OUT
- 3.5mm output jack
- Connects to external modular gear
Cutoff Frequency
R35 + C21 form low-pass filter:
fc = 1 / (2π × 100kΩ × 470pF)
fc ≈ 3.4 kHz
Purpose: Smooth CV changes, remove high-frequency noise
Channel B: LFO CV Output (IC9A)
Buffer Stage (IC9A - TL072P)
Configuration: Unity gain voltage follower
Op-amp pins:
- Pin 3 (+): Input from DACA
- Pin 2 (-): Feedback from output
- Pin 1 (Output): To LFO_CV_OUT jack
Signal Path Components
Identical architecture to Channel A:
R31 (24.9kΩ): V_BIAS connection
C20 (0.1µF): AC coupling capacitor
- Larger than Channel A (100nF vs 470pF)
- Passes lower frequencies
- Better for slow LFO sweeps
C19 (470pF): High-frequency filter
R32 (100kΩ): Feedback resistor
R33 (220Ω): Output protection
How It Works:
Independent Channel Operation
Element CV (Channel A):
1. Teensy sends data via SPI
2. MCP9422 updates DACB output
3. Voltage: 0 to 2.5V
4. Buffer (IC9B) isolates and filters
5. Output: Clean CV at ELEMENT_CV_OUT jack
LFO CV (Channel B):
1. Teensy sends data via SPI (different command)
2. MCP9422 updates DACA output
3. Voltage: 0 to 2.5V
4. Buffer (IC9A) isolates and filters
5. Output: Clean CV at LFO_CV_OUT jack): LFO_CV_OUT
Result: Professional CV outputs for expanding the Archean with external modular gear - endless patching possibilities!
Filter
Voltage Controlled Filter based on the classic Korg MS-20 design - one of the most iconic synthesizer filters ever made.
Purpose: Aggressive, resonant filtering with character and attitude.
What is the MS-20 Filter?
Historic context:
- Original design from 1978 Korg MS-20 synthesizer
- Famous for aggressive, screaming resonance
- High-pass and low-pass sections
- Capable of self-oscillation
- Used on countless records
Character: Raw, aggressive, in-your-face analog sound
Stage 1: High-Pass Filter (IC14A - TL074P)
First stage removes low frequencies
Key components:
- C31 (4.7nF): HPF capacitor
- R70 (10kΩ): Sets HPF cutoff
- VR11 (pot): Cutoff control
Purpose:
- Removes rumble and DC
- Shapes low-end response
- Can thin out bass for certain sounds
Typical cutoff: ~20 Hz to ~500 Hz (adjustable)
Stage 2: Voltage Controlled Section (IC13A, IC13B - LM13700N)
LM13700N dual transconductance amplifier
What is a transconductance amp?
- Voltage-controlled resistor/amplifier
- Current output proportional to voltage input
- Perfect for voltage-controlled filters
Components:
- R54, R55 (220Ω): Bias resistors
- R56 (220Ω): Sets transconductance
- Control inputs (pins 1, 16): CV from external sources
Function:
- Control voltage changes filter cutoff frequency
- Linear voltage-to-frequency response
- Classic analog VCF behavior
Stage 3: Low-Pass Filter (IC14C - TL074P)
Main filtering stage
Key components:
- R62, R57 (10kΩ): Input mixing
- C30 (1nF), C33 (0.47µF): Filter capacitors
- R63 (10kΩ): Resonance feedback path
- VR10 (pot): Resonance control
Configuration: 2-pole (12 dB/octave) with resonance feedback
Resonance network:
Output → R63 → Back to input
More feedback = More resonance = Peak at cutoff
Self-oscillation: At maximum resonance, filter can oscillate and produce a sine wave!
Stage 4: VCA Section (LED2, LED3 + Q5, Q6)
Optocoupler-based voltage controlled amplifier
Components:
- LED2, LED3: Vactrol-style optocouplers
- Q5, Q6: Control transistors (likely 2N3904)
- R68, R69 (1.8kΩ): LED current limiting
- R66 (100kΩ): Feedback resistor
How it works:
Control voltage → Q5/Q6 conduct → Current through LEDs
LED brightness → Photoresistor changes → Signal gain changes
More voltage = Brighter = More gain
Control sources:
- ADSR envelope (volume shaping)
- Distance sensor CV (gesture control)
- External CV inputs
Stage 5: Output Buffer (IC14D - TL074P)
Final stage
Components:
- R67 (100kΩ): Feedback
- C32 (1nF): Frequency compensation
- R64, R65 (10kΩ): Output network
Purpose:
- Buffers filter output
- Provides clean drive signal
- Isolates filter from load
User Controls
VR11 - Cutoff Frequency
- Adjusts filter cutoff (brightness)
- Wide range: bassy to bright
- Most expressive control
VR10 - Resonance
- Emphasis at cutoff frequency
- Low: Smooth rolloff
- High: Sharp peak, self-oscillation
- Classic "screaming" MS-20 sound
CV Inputs (J17)
- External CV jacks
- Control cutoff via voltage
- Keyboard tracking, envelopes, LFOs
MS-20 Filter Characteristics
What makes it special:
- Aggressive resonance - Can scream and howl
- Self-oscillation - Becomes a sine wave oscillator
- HPF + LPF - Dual filtering for complex timbres
- Raw character - Slightly dirty, aggressive, alive
- Musical distortion - Overdrives naturally at high resonance
NOT a smooth Moog-style filter - This is wild and aggressive!
Purpose: Aggressive, resonant filtering with character and attitude.
What is the MS-20 Filter?
Historic context:
- Original design from 1978 Korg MS-20 synthesizer
- Famous for aggressive, screaming resonance
- High-pass and low-pass sections
- Capable of self-oscillation
- Used on countless records
Character: Raw, aggressive, in-your-face analog sound
Stage 1: High-Pass Filter (IC14A - TL074P)
First stage removes low frequencies
Key components:
- C31 (4.7nF): HPF capacitor
- R70 (10kΩ): Sets HPF cutoff
- VR11 (pot): Cutoff control
Purpose:
- Removes rumble and DC
- Shapes low-end response
- Can thin out bass for certain sounds
Typical cutoff: ~20 Hz to ~500 Hz (adjustable)
Stage 2: Voltage Controlled Section (IC13A, IC13B - LM13700N)
LM13700N dual transconductance amplifier
What is a transconductance amp?
- Voltage-controlled resistor/amplifier
- Current output proportional to voltage input
- Perfect for voltage-controlled filters
Components:
- R54, R55 (220Ω): Bias resistors
- R56 (220Ω): Sets transconductance
- Control inputs (pins 1, 16): CV from external sources
Function:
- Control voltage changes filter cutoff frequency
- Linear voltage-to-frequency response
- Classic analog VCF behavior
Stage 3: Low-Pass Filter (IC14C - TL074P)
Main filtering stage
Key components:
- R62, R57 (10kΩ): Input mixing
- C30 (1nF), C33 (0.47µF): Filter capacitors
- R63 (10kΩ): Resonance feedback path
- VR10 (pot): Resonance control
Configuration: 2-pole (12 dB/octave) with resonance feedback
Resonance network:
Output → R63 → Back to input
More feedback = More resonance = Peak at cutoff
Self-oscillation: At maximum resonance, filter can oscillate and produce a sine wave!
Stage 4: VCA Section (LED2, LED3 + Q5, Q6)
Optocoupler-based voltage controlled amplifier
Components:
- LED2, LED3: Vactrol-style optocouplers
- Q5, Q6: Control transistors (likely 2N3904)
- R68, R69 (1.8kΩ): LED current limiting
- R66 (100kΩ): Feedback resistor
How it works:
Control voltage → Q5/Q6 conduct → Current through LEDs
LED brightness → Photoresistor changes → Signal gain changes
More voltage = Brighter = More gain
Control sources:
- ADSR envelope (volume shaping)
- Distance sensor CV (gesture control)
- External CV inputs
Stage 5: Output Buffer (IC14D - TL074P)
Final stage
Components:
- R67 (100kΩ): Feedback
- C32 (1nF): Frequency compensation
- R64, R65 (10kΩ): Output network
Purpose:
- Buffers filter output
- Provides clean drive signal
- Isolates filter from load
User Controls
VR11 - Cutoff Frequency
- Adjusts filter cutoff (brightness)
- Wide range: bassy to bright
- Most expressive control
VR10 - Resonance
- Emphasis at cutoff frequency
- Low: Smooth rolloff
- High: Sharp peak, self-oscillation
- Classic "screaming" MS-20 sound
CV Inputs (J17)
- External CV jacks
- Control cutoff via voltage
- Keyboard tracking, envelopes, LFOs
MS-20 Filter Characteristics
What makes it special:
- Aggressive resonance - Can scream and howl
- Self-oscillation - Becomes a sine wave oscillator
- HPF + LPF - Dual filtering for complex timbres
- Raw character - Slightly dirty, aggressive, alive
- Musical distortion - Overdrives naturally at high resonance
NOT a smooth Moog-style filter - This is wild and aggressive!
VCA
Voltage Controlled Amplifier - Controls audio output level using CV from ADSR envelope, knob and CV.
Purpose: Shapes volume over time (envelope) and allows expressive gesture control.
Stage 1: Control Voltage Mixer (IC15A - TL072P)
Inputs:
- ADSR: Envelope voltage from DAC
- J18 (Jack): External CV input
- VR2 (pot): Manual level control
Components:
- R73, R74, R75 (100kΩ): Mix resistors
- R76 (10kΩ): Feedback resistor
Function: Combines multiple control sources into single CV
Formula:
Output CV = (ADSR + External CV + Manual) / 3
Stage 2: VCA Core (Q7, Q8 - Transistors)
Configuration: Transistor-based voltage controlled attenuator
Q7, Q8: Control transistors (BC548 or similar)
How it works:
Higher control voltage → More transistor conduction → More signal passes
Lower control voltage → Less conduction → Signal attenuated
Key components:
- R71 (100kΩ): Bias resistor
- R80, R81 (10kΩ): Sets gain range
- R77 (10kΩ): Feedback
Result: Audio signal amplitude controlled by CV
Stage 3: Output Buffer (IC15B - TL072P)
Configuration: Non-inverting amplifier
Components:
- R80, R81 (10kΩ): Input resistors
- R83 (470kΩ): Feedback resistor
- R84 (1kΩ): Output resistor
Gain calculation:
Gain = 1 + (R83 / R80)
Gain = 1 + (470k / 10k)
Gain = 1 + 47 = 48× (about 34dB)
Purpose:
- Amplifies VCA output to usable level
- Buffers load from output stage
- Provides strong drive signal
Control Sources
Three CV inputs mixed:
1. ADSR envelope - Time-varying level (attack, decay, sustain, release)
2. External CV (U18 jack) - External modulation sources
3. Manual control (VR2) - User sets base level
Flexible volume control!
Key Features:
- Multiple CV inputs - ADSR, external, manual
- Transistor VCA - Smooth, musical response
- High gain output - 48× amplification
- Buffered output - Drives next stage easily
Result: Expressive volume control that brings the synthesizer to life with dynamic amplitude changes!
Purpose: Shapes volume over time (envelope) and allows expressive gesture control.
Stage 1: Control Voltage Mixer (IC15A - TL072P)
Inputs:
- ADSR: Envelope voltage from DAC
- J18 (Jack): External CV input
- VR2 (pot): Manual level control
Components:
- R73, R74, R75 (100kΩ): Mix resistors
- R76 (10kΩ): Feedback resistor
Function: Combines multiple control sources into single CV
Formula:
Output CV = (ADSR + External CV + Manual) / 3
Stage 2: VCA Core (Q7, Q8 - Transistors)
Configuration: Transistor-based voltage controlled attenuator
Q7, Q8: Control transistors (BC548 or similar)
How it works:
Higher control voltage → More transistor conduction → More signal passes
Lower control voltage → Less conduction → Signal attenuated
Key components:
- R71 (100kΩ): Bias resistor
- R80, R81 (10kΩ): Sets gain range
- R77 (10kΩ): Feedback
Result: Audio signal amplitude controlled by CV
Stage 3: Output Buffer (IC15B - TL072P)
Configuration: Non-inverting amplifier
Components:
- R80, R81 (10kΩ): Input resistors
- R83 (470kΩ): Feedback resistor
- R84 (1kΩ): Output resistor
Gain calculation:
Gain = 1 + (R83 / R80)
Gain = 1 + (470k / 10k)
Gain = 1 + 47 = 48× (about 34dB)
Purpose:
- Amplifies VCA output to usable level
- Buffers load from output stage
- Provides strong drive signal
Control Sources
Three CV inputs mixed:
1. ADSR envelope - Time-varying level (attack, decay, sustain, release)
2. External CV (U18 jack) - External modulation sources
3. Manual control (VR2) - User sets base level
Flexible volume control!
Key Features:
- Multiple CV inputs - ADSR, external, manual
- Transistor VCA - Smooth, musical response
- High gain output - 48× amplification
- Buffered output - Drives next stage easily
Result: Expressive volume control that brings the synthesizer to life with dynamic amplitude changes!
Distortion
Waveshaper Distortion Stage - Adds harmonic saturation and clipping to audio signals using diode-based soft clipping.
Purpose: Transforms clean audio into harmonically rich, overdriven tones by clipping signal peaks.
Stage 1: Input Coupling & DC Blocking (C34)
Component:
- C34 (10 µF): 1. capacitor
Function:
- Blocks DC voltage from previous stage
- Allows only AC audio signal to pass
- Sets low-frequency cutoff with op-amp input impedance
Stage 2: Op-Amp Gain Stage (IC16A - TL072P)
Configuration: Inverting amplifier with frequency shaping
Key components:
- Feedback network: C35 (470pF), C36 (47nF)
- No visible resistors shown (assumed off-page or default values)
How it works:
- Amplifies signal to drive diodes into clipping
- Capacitors C35 and C36 shape frequency response
- C35 (small value): Affects high-frequency gain/response
- C36 (larger value): Sets mid/low-frequency gain characteristics
Result: Boosts signal to appropriate level for diode clipping
Stage 3: Diode Clipping (D3, D4 - 1N4148)
Configuration: Symmetrical soft clipping in feedback path
Diodes: 1N4148 (general purpose switching diodes)
How it works:
- Signal peaks exceeding diode forward voltage (~0.7V) get clipped
- D3 clips positive peaks
- D4 clips negative peaks
- Creates symmetrical waveform distortion
- Soft clipping creates musical, harmonically rich saturation
Effect:
- Gentle rounding of waveform peaks
- Adds even and odd harmonics
- More musical than hard clipping
- Dynamic response: louder signals clip more
Stage 4: Output Coupling (C37)
Component:
- C37 (10 µF): 1. capacitor
Function:
- Removes any DC offset created by clipping stage
- Couples distorted signal to next stage (reverb)
- Matches input coupling for symmetrical frequency response
Frequency Shaping Network
Capacitor functions:
- C35 (470pF): High-frequency emphasis or roll-off control
- C36 (47nF): Primary tone shaping, affects mid-range response
- Together they create a characteristic distortion EQ curve
Typical behavior:
· Boosts mids for "punchy" distortion
· Controls high-frequency fizz
· Creates musical emphasis in guitar-friendly frequencies
Diode Characteristics
1N4148 diodes:
- Forward voltage: ~0.7V
- Fast switching speed (important for audio)
- Creates "soft knee" clipping
- More musical than LED or silicon clipping alternatives
Clipping symmetry:
- Symmetrical clipping: Balanced even/odd harmonics
- Creates "tube-like" warmth
- Less harsh than asymmetrical clipping
Power Supply
Op-amp (TL072P): Powered by ±12V (dual supply)
- Allows signal swing above and below ground
- Provides headroom before clipping
- Clean power prevents unwanted noise in distortion
Result: Transforms clean synth tones into harmonically rich, expressive distorted sounds perfect for leads, basses, and textural elements!
Purpose: Transforms clean audio into harmonically rich, overdriven tones by clipping signal peaks.
Stage 1: Input Coupling & DC Blocking (C34)
Component:
- C34 (10 µF): 1. capacitor
Function:
- Blocks DC voltage from previous stage
- Allows only AC audio signal to pass
- Sets low-frequency cutoff with op-amp input impedance
Stage 2: Op-Amp Gain Stage (IC16A - TL072P)
Configuration: Inverting amplifier with frequency shaping
Key components:
- Feedback network: C35 (470pF), C36 (47nF)
- No visible resistors shown (assumed off-page or default values)
How it works:
- Amplifies signal to drive diodes into clipping
- Capacitors C35 and C36 shape frequency response
- C35 (small value): Affects high-frequency gain/response
- C36 (larger value): Sets mid/low-frequency gain characteristics
Result: Boosts signal to appropriate level for diode clipping
Stage 3: Diode Clipping (D3, D4 - 1N4148)
Configuration: Symmetrical soft clipping in feedback path
Diodes: 1N4148 (general purpose switching diodes)
How it works:
- Signal peaks exceeding diode forward voltage (~0.7V) get clipped
- D3 clips positive peaks
- D4 clips negative peaks
- Creates symmetrical waveform distortion
- Soft clipping creates musical, harmonically rich saturation
Effect:
- Gentle rounding of waveform peaks
- Adds even and odd harmonics
- More musical than hard clipping
- Dynamic response: louder signals clip more
Stage 4: Output Coupling (C37)
Component:
- C37 (10 µF): 1. capacitor
Function:
- Removes any DC offset created by clipping stage
- Couples distorted signal to next stage (reverb)
- Matches input coupling for symmetrical frequency response
Frequency Shaping Network
Capacitor functions:
- C35 (470pF): High-frequency emphasis or roll-off control
- C36 (47nF): Primary tone shaping, affects mid-range response
- Together they create a characteristic distortion EQ curve
Typical behavior:
· Boosts mids for "punchy" distortion
· Controls high-frequency fizz
· Creates musical emphasis in guitar-friendly frequencies
Diode Characteristics
1N4148 diodes:
- Forward voltage: ~0.7V
- Fast switching speed (important for audio)
- Creates "soft knee" clipping
- More musical than LED or silicon clipping alternatives
Clipping symmetry:
- Symmetrical clipping: Balanced even/odd harmonics
- Creates "tube-like" warmth
- Less harsh than asymmetrical clipping
Power Supply
Op-amp (TL072P): Powered by ±12V (dual supply)
- Allows signal swing above and below ground
- Provides headroom before clipping
- Clean power prevents unwanted noise in distortion
Result: Transforms clean synth tones into harmonically rich, expressive distorted sounds perfect for leads, basses, and textural elements!
Reverberation
Hybrid Spring + Digital Reverb - Combines a classic Accutronics-type spring reverb tank with a PT2399-based digital delay for extended, lush reverberation effects.
Purpose: Adds spacious, atmospheric depth to the synthesizer's output with characteristic spring "drip" and controllable digital decay.
Stage 1: Input and Spring Driver (TL072 sections)
Configuration: Multiple op-amp stages for mixing and driving the tank
Inputs:
- Main audio input (from previous stage)
- Possible feedback or mix points
Components:
- Various 100kΩ, 10kΩ resistors for mixing
- Capacitors for AC coupling and filtering
- TL072 op-amps: Low-noise dual op-amps handling signal routing and amplification
Function: Conditions the input signal and drives the spring tank transducer
How it works:
- Signal is buffered and amplified
- Drives the input coil of the spring tank, causing mechanical vibrations in the springs
- Springs create natural delayed reflections with characteristic metallic timbre
Stage 2: Spring Tank (Physical Reverb)
Type: Accutronics/Belton-style dual or triple-spring tank
Key characteristics:
- Input transducer converts electrical signal to mechanical waves
- Springs delay and diffuse the sound
- Output transducer picks up the reverberated vibrations
Result: Classic "splashy" analog spring reverb with physical presence and boing on accents
Stage 3: Recovery and Digital Extension (TL072 + PT2399)
Configuration:
- Recovery: High-gain op-amp stage (TL072)
- Digital core: PT2399 echo processor IC
Components:
- Recovery: 100kΩ/10kΩ resistors setting high gain
- PT2399: External resistors/capacitors control delay time and feedback
- Multiple filtering caps for tone shaping
- Feedback loops around PT2399 for repeating echoes
How it works:
- Weak signal from tank output is amplified in recovery stage
- Fed into PT2399 which adds longer digital delays
- Feedback creates decaying repeats, extending reverb tail beyond spring limits
- Low-pass filtering in PT2399 loop reduces high-frequency harshness for smoother decay
Result: Combines short spring reflections with longer digital tail for room-like reverb
Stage 4: Output Mixer and Buffering (TL072 sections)
Configuration: Summing amplifier mixing dry and wet signals
Components:
- Mixing resistors (typically 100kΩ range)
- Final buffer op-amp for low-impedance output
Function:
- Blends original dry signal with processed wet reverb
- Controls overall reverb amount
- Buffers for driving next stage or output
Control Sources
Likely controls (based on typical designs and visible pots):
1. Reverb mix/level - Balances dry/wet
2. Decay/time - Adjusts PT2399 delay resistance for longer/shorter tail
3. Tone/damping - Filters feedback loop for brighter or darker reverb
Classic spring character with extended digital depth!
Key Features
- Hybrid design - Authentic spring + digital extension
- PT2399 core - Long, controllable decay tails
- Multiple TL072 stages - Clean drive, recovery, and mixing
- Physical tank - Iconic splash and movement sensitivity
Result: Rich, evocative reverb that adds vast primordial atmosphere to the Archean synthesizer's soundscapes!
Purpose: Adds spacious, atmospheric depth to the synthesizer's output with characteristic spring "drip" and controllable digital decay.
Stage 1: Input and Spring Driver (TL072 sections)
Configuration: Multiple op-amp stages for mixing and driving the tank
Inputs:
- Main audio input (from previous stage)
- Possible feedback or mix points
Components:
- Various 100kΩ, 10kΩ resistors for mixing
- Capacitors for AC coupling and filtering
- TL072 op-amps: Low-noise dual op-amps handling signal routing and amplification
Function: Conditions the input signal and drives the spring tank transducer
How it works:
- Signal is buffered and amplified
- Drives the input coil of the spring tank, causing mechanical vibrations in the springs
- Springs create natural delayed reflections with characteristic metallic timbre
Stage 2: Spring Tank (Physical Reverb)
Type: Accutronics/Belton-style dual or triple-spring tank
Key characteristics:
- Input transducer converts electrical signal to mechanical waves
- Springs delay and diffuse the sound
- Output transducer picks up the reverberated vibrations
Result: Classic "splashy" analog spring reverb with physical presence and boing on accents
Stage 3: Recovery and Digital Extension (TL072 + PT2399)
Configuration:
- Recovery: High-gain op-amp stage (TL072)
- Digital core: PT2399 echo processor IC
Components:
- Recovery: 100kΩ/10kΩ resistors setting high gain
- PT2399: External resistors/capacitors control delay time and feedback
- Multiple filtering caps for tone shaping
- Feedback loops around PT2399 for repeating echoes
How it works:
- Weak signal from tank output is amplified in recovery stage
- Fed into PT2399 which adds longer digital delays
- Feedback creates decaying repeats, extending reverb tail beyond spring limits
- Low-pass filtering in PT2399 loop reduces high-frequency harshness for smoother decay
Result: Combines short spring reflections with longer digital tail for room-like reverb
Stage 4: Output Mixer and Buffering (TL072 sections)
Configuration: Summing amplifier mixing dry and wet signals
Components:
- Mixing resistors (typically 100kΩ range)
- Final buffer op-amp for low-impedance output
Function:
- Blends original dry signal with processed wet reverb
- Controls overall reverb amount
- Buffers for driving next stage or output
Control Sources
Likely controls (based on typical designs and visible pots):
1. Reverb mix/level - Balances dry/wet
2. Decay/time - Adjusts PT2399 delay resistance for longer/shorter tail
3. Tone/damping - Filters feedback loop for brighter or darker reverb
Classic spring character with extended digital depth!
Key Features
- Hybrid design - Authentic spring + digital extension
- PT2399 core - Long, controllable decay tails
- Multiple TL072 stages - Clean drive, recovery, and mixing
- Physical tank - Iconic splash and movement sensitivity
Result: Rich, evocative reverb that adds vast primordial atmosphere to the Archean synthesizer's soundscapes!
Keyboard sensors
Capacitive Touch Interface (Keyboard) - Uses two MPR121 proximity capacitive touch sensor ICs to provide 22 touch-sensitive keys/pads.
Stage 1: MPR121 Capacitive Touch Sensors (S1 & S2)
Configuration: Dedicated 12-channel capacitive touch controller ICs
Key pins:
- IRQ (pin 3): Interrupt request – pulled low when touch detected
- SDA (pin 6): I²C data line
- SCL (pin 5): I²C clock line
- ADDR (pin 7): Address selection pin
- 3.3V and GND: Power supply
Address configuration:
- S1: ADDR pin not connected or pulled low → I²C address 0x5A (default)
- S2: ADDR pin connected to 3.3V → I²C address 0x5B
Function:
- Each of the 12 electrodes (pins 0–11) measures tiny capacitance changes when a finger approaches
- On-chip baseline tracking and noise filtering
- Detects touch/release and can report touch strength (useful for velocity or aftertouch)
Result: Reliable, adjustable-sensitivity touch detection
Stage 2: I²C Bus Connection
Components:
- Shared SCL and SDA lines connected to both MPR121 ICs
- Teensy 4.0 pins (typically 19 for SCL, 18 for SDA on standard breakout)
How it works:
- Teensy acts as I²C master
- Polls or uses IRQ interrupts to read touch data from both chips using their distinct addresses (0x5A and 0x5B)
Stage 3: Interrupt and Proximity Inputs (Q3, Q4, R36, R37, R38)
Extra connections:
- MPR121_IRQ (from a pin or combined IRQ) → base of Q3 via R36 (4.7kΩ)
- Q3 (NPN transistor): Collector pulled to 3.3V via R37 (10kΩ), emitter to ground
- Q4 (NPN transistor): Similar setup with R38 (10kΩ)
Function:
- Likely combines or inverts the open-drain IRQ outputs from both MPR121s
- Provides a clean active-low or active-high interrupt signal to a Teensy GPIO pin
- Allows efficient event-driven reading (Teensy only checks I²C bus when touch state changes)
Control Sources
1. Touch electrodes (22 total): Primary playing interface – note on/off
2. Configurable sensitivity: Set via I²C registers in firmware (filtering, baseline, charge current)
Key Features
- 24 touch channels – Full two-octave+ touch keyboard
- Dual MPR121 – Smart addressing via ADDR pin
- IRQ interrupt system – Efficient real-time response
- Velocity/aftertouch capable – From touch data values
- I²C interface – Simple two-wire connection to Teensy
Result: A responsive, expressive, and futuristic touch interface that gives the Archean synthesizer its distinctive playable character!
Stage 1: MPR121 Capacitive Touch Sensors (S1 & S2)
Configuration: Dedicated 12-channel capacitive touch controller ICs
Key pins:
- IRQ (pin 3): Interrupt request – pulled low when touch detected
- SDA (pin 6): I²C data line
- SCL (pin 5): I²C clock line
- ADDR (pin 7): Address selection pin
- 3.3V and GND: Power supply
Address configuration:
- S1: ADDR pin not connected or pulled low → I²C address 0x5A (default)
- S2: ADDR pin connected to 3.3V → I²C address 0x5B
Function:
- Each of the 12 electrodes (pins 0–11) measures tiny capacitance changes when a finger approaches
- On-chip baseline tracking and noise filtering
- Detects touch/release and can report touch strength (useful for velocity or aftertouch)
Result: Reliable, adjustable-sensitivity touch detection
Stage 2: I²C Bus Connection
Components:
- Shared SCL and SDA lines connected to both MPR121 ICs
- Teensy 4.0 pins (typically 19 for SCL, 18 for SDA on standard breakout)
How it works:
- Teensy acts as I²C master
- Polls or uses IRQ interrupts to read touch data from both chips using their distinct addresses (0x5A and 0x5B)
Stage 3: Interrupt and Proximity Inputs (Q3, Q4, R36, R37, R38)
Extra connections:
- MPR121_IRQ (from a pin or combined IRQ) → base of Q3 via R36 (4.7kΩ)
- Q3 (NPN transistor): Collector pulled to 3.3V via R37 (10kΩ), emitter to ground
- Q4 (NPN transistor): Similar setup with R38 (10kΩ)
Function:
- Likely combines or inverts the open-drain IRQ outputs from both MPR121s
- Provides a clean active-low or active-high interrupt signal to a Teensy GPIO pin
- Allows efficient event-driven reading (Teensy only checks I²C bus when touch state changes)
Control Sources
1. Touch electrodes (22 total): Primary playing interface – note on/off
2. Configurable sensitivity: Set via I²C registers in firmware (filtering, baseline, charge current)
Key Features
- 24 touch channels – Full two-octave+ touch keyboard
- Dual MPR121 – Smart addressing via ADDR pin
- IRQ interrupt system – Efficient real-time response
- Velocity/aftertouch capable – From touch data values
- I²C interface – Simple two-wire connection to Teensy
Result: A responsive, expressive, and futuristic touch interface that gives the Archean synthesizer its distinctive playable character!
Noise Generator
White Noise Generator - Classic avalanche breakdown noise source using a single transistor, amplified and buffered for synthesizer use.
Purpose: Provides a broadband random signal for percussion (hiss, snares), wind/sea effects, or as a modulation source (via sample & hold for random voltages).
Stage 1: Avalanche Noise Source (Q2 - NPN Transistor)
Configuration: Reverse-biased base-emitter junction
Components:
- Q2: Likely 2N3904 or similar general-purpose NPN
- R45 (100kΩ): Current-limiting resistor from +12V
How it works:
- Emitter connected toward +12V via R45
- avalanche effect generates intense broadband (white) noise at the emitter
Result: Raw microscopic noise voltage (~mV level) with flat frequency spectrum
Note: Noise amplitude varies between individual transistors; some are "noisier" than others
Stage 2: High-Gain Amplifier (IC10A - TL072P)
Configuration: Non-inverting op-amp amplifier
Components:
- R48 (270kΩ): Feedback resistor
- R47 (10kΩ pot): Variable input/gain resistor to ground
- C22 (470nF): AC coupling/decoupling capacitor
Gain calculation:
Maximum gain (pot at 0Ω) ≈ 1 + (R48 / 0) → very high (limited by op-amp)
Typical gain range: Extremely high (1000+×) down to lower via pot adjustment
Function:
- Massively amplifies tiny noise from Q2 emitter
- Pot R47 adjusts overall noise level/output amplitude
- TL072 provides low-noise, high-impedance input suitable for this
Result: Brings noise up to usable audio/modulation levels
Stage 3: Low-Pass Filtering and Output (C23 + J10)
Components:
- C23 (47nF): High-frequency roll-off capacitor
- J10: Output jack (PS1, PS2, PS3 pins – likely tip/ring/sleeve)
Function:
- C23 forms a simple RC low-pass filter with output impedance
- Gently tames ultra-high frequencies for smoother, more musical white noise
- Direct output via jack for patching
Classic, simple, and effective randomness!
Key Features:
- Transistor avalanche – True analog white noise source
- Extremely high gain stage – Amplifies microvolt noise to line level
- Level control pot – Adjustable output intensity
- Simple filtering – Tamed highs for synth-friendly noise
Result: Essential raw material for percussive hits, atmospheric textures, and unpredictable modulation in the Archean synthesizer!
Purpose: Provides a broadband random signal for percussion (hiss, snares), wind/sea effects, or as a modulation source (via sample & hold for random voltages).
Stage 1: Avalanche Noise Source (Q2 - NPN Transistor)
Configuration: Reverse-biased base-emitter junction
Components:
- Q2: Likely 2N3904 or similar general-purpose NPN
- R45 (100kΩ): Current-limiting resistor from +12V
How it works:
- Emitter connected toward +12V via R45
- avalanche effect generates intense broadband (white) noise at the emitter
Result: Raw microscopic noise voltage (~mV level) with flat frequency spectrum
Note: Noise amplitude varies between individual transistors; some are "noisier" than others
Stage 2: High-Gain Amplifier (IC10A - TL072P)
Configuration: Non-inverting op-amp amplifier
Components:
- R48 (270kΩ): Feedback resistor
- R47 (10kΩ pot): Variable input/gain resistor to ground
- C22 (470nF): AC coupling/decoupling capacitor
Gain calculation:
Maximum gain (pot at 0Ω) ≈ 1 + (R48 / 0) → very high (limited by op-amp)
Typical gain range: Extremely high (1000+×) down to lower via pot adjustment
Function:
- Massively amplifies tiny noise from Q2 emitter
- Pot R47 adjusts overall noise level/output amplitude
- TL072 provides low-noise, high-impedance input suitable for this
Result: Brings noise up to usable audio/modulation levels
Stage 3: Low-Pass Filtering and Output (C23 + J10)
Components:
- C23 (47nF): High-frequency roll-off capacitor
- J10: Output jack (PS1, PS2, PS3 pins – likely tip/ring/sleeve)
Function:
- C23 forms a simple RC low-pass filter with output impedance
- Gently tames ultra-high frequencies for smoother, more musical white noise
- Direct output via jack for patching
Classic, simple, and effective randomness!
Key Features:
- Transistor avalanche – True analog white noise source
- Extremely high gain stage – Amplifies microvolt noise to line level
- Level control pot – Adjustable output intensity
- Simple filtering – Tamed highs for synth-friendly noise
Result: Essential raw material for percussive hits, atmospheric textures, and unpredictable modulation in the Archean synthesizer!